WASHINGTON (Reuters) - President Barack Obama said on Tuesday that Mexico's capture of the leader of a notorious drug cartel provided reassurance that President Enrique Pena Nieto's commitment to fighting drug trafficking was solid.

"What it shows is that the new administration of President Pena Nieto is serious about continuing the efforts to break up these transnational drug operations," Obama said in an interview with Univision's Los Angeles affiliate. "And there had been some question about that, I think early on during his campaign, and immediately after his election."

Pena Nieto caused some concern in the United States by dialing back the aggressive campaign against drug trafficking pursued by his predecessor, Felipe Calderon, in favor of a policy emphasizing reducing violence.

Under the new approach, Pena Nieto reined in the wide latitude Calderon gave the U.S. government in working with Mexican officials, replacing it with a single point of contact, Mexico's Ministry of Interior.

Obama endorsed the policy in May during a visit to Mexico, saying it was up to the Mexican people to determine their own security structures.

But he said on Tuesday that Monday's arrest of Miguel Angel Trevino, also called Z-40, the leader of the Zetas drug cartel, offered proof that Pena Nieto's approach could be effective.

"He indicated to me that he recognizes the need to deal with these transnational drug cartels in a serious way," Obama said. "And I think this is evidence of it."

Obama said that clamping down on the international drug trade in Mexico was in the U.S. interest, and that Washington supported Mexican efforts. The United States recognizes for its part that it has a role to play in curtailing the market for drugs at home and the flow of guns to Mexico.

"We want to make sure that they know that we're a partner," Obama said. "It also means though we have to continue doing our part here in the United States to reduce demand, and reduce the flow of guns and cash down south."